1. Field of the Invention
The present invention relates to an electrophotographic device such as a copying machine of the electrophotographic system equipped with an exposure optical system for transmitting a light reflected on an original to a photosensitive body and a screen filter disposed on the optical path for dividing the light exposing the photosensitive body linearly.
2. Description of the Prior Art
The screen process has been known as a method for improving gradation of an image in a copying machine of the electrophotographic system. In this process, a screen on which light transmitting portions and light non-transmitting portions in the shape of strips are alternately formed is provided near a photosensitive body. A light reflected on an original, which is divided linearly by the screen, is emitted onto the photosensitive body for exposure. Thus the copied image is given gradation in accordance with the width of the strips on the screen.
In this process, an accuracy of the gap between the screen and the photosensitive body must be very precise. This process is effective since a shadow of the screen on the photosensitive body can be wide or narrow, depending upon the quantity of light for image exposure. Therefore, it is very important to keep the gap from the photosensitive body with an accuracy of 0.2 mm or less.
Moreover, these pitches on the screen are revealed on the copied image, therefore, it is necessary to make the pitches typically 120 lines/inch or less in order to make them unnoticeable. The width of the light transmitting portions, which depends upon the distance from the photosensitive body, is most preferably 50 to 70 .mu.m and the gap is most preferably 0.8 to 1.5 mm.
A film with a thickness of about 100 .mu.m made by the electrophotographic system is generally used as the screen because it requires a precise accuracy in both the width and the pitch.
The above-mentioned screen is mounted with a strong tension in the transverse direction of the copying machine with its screen surface opposite the surface of the photosensitive body below so as to maintain the gap from the photosensitive body. Moreover, a device and the like to strengthen the ends of the film and to transfer the tension equally in all directions is provided so that a strong force can be given to the film of a material with small tear resistance.
However, in the above-mentioned disposition, it is disadvantageously difficult to keep uniform the gap from the surface of the photosensitive body, since the center portion of the screen in the transverse direction of the copying machine tends yield due to its own weight. Thus the gap in the center portion is narrower than those in the end portions, thereby causing a problem of a fogging in the copied image, that is, a phenomenon that toner sticks to a portion of the copying paper which must remain white.
FIG. 5 shows a conventional color copying machine of the electrophotographic system. In the movable area of a photosensitive belt 70 shown by an arrow B, a yellow developing vessel 40, a magenta developing vessel 50, a cyanogen developing vessel 60 and a black developing vessel 30 are provided adjacent in this order from the upstream of the movement of the belt. In this color copying machine, an original on a placing table is scanned by an optical system 200 three times. At each time a light reflected on the original is slit exposed through a color separation filter onto the photosensitive belt 70. Thus electrostatic latent images are formed in complementary colors of yellow, magenta and cyanogen. Every time an electrostatic latent image is formed, yellow toner, magenta toner and cyanogen toner are respectively supplied in this order from the yellow developing vessel 40, the magenta developing vessel 50 and the cyan developing vessel 60, thereby superimposing these images on a transfer belt 100 which is pressure welded to the photosensitive belt 70.
After superimposing, a copying paper is transferred from a paper cassette 80 to a transfer roller 220, where a toner image is transferred onto the copying paper. The image on the copying paper after the transferring process is fixed by a fixing device and then fed out of the copying machine.
In the case of monochrome copying, only black toner in the black developing vessel 30 is used.
The developing device in which the developing vessels are provided adjacent as above has a shortcoming in that color contamination tends to be caused by scattered toner from the black developing vessel 30 in the yellow developing vessel 40 the magenta developing vessel 50 and especially the cyan developing vessel 60 which is next to the black developing vessel 30. The color contamination area is shown by a reference P in FIG. 5.
The color contamination reduces the color reproduction in color copying, thereby spoiling the quality of the image, which will be a dark cloudy image as a whole.
Additionally, when each of the developing vessels is provided on the same side of the photosensitive belt 70, it is difficult to mount all the developing vessels in a predetermined space as the required mounting space is too large. The same problem occurs also in a color copying machine utilizing a photosensitive drum as a photosensitive body.
Such a problem in, for example, a color copying machine utilizing a photosensitive drum, can be solved by making the diameter of the photosensitive drum large enough to make the space around the drum also large. Then it is possible to secure the mounting space of the developing vessels. However, this causes another problem that the color copying machine itself becomes large-sized.
On the other hand, small developing vessels are advantageous in the mounting space but cause other problems such as slow developing speed, short life time of developers and reduced developing performance.
Due to the above-mentioned reasons, it is desired to realize a compact copying machine in which developing vessels are effectively mounted without wasteful space.